1. Field of the Invention
The present invention relates to semiconductor packages, and more particularly to packages in which a transistor or other die is mounted together with an insulated lead frame on a metallic, electrically conductive substrate which acts as a heat spreader as well as an electrical connection for the die.
2. History of the Prior Art
It is known in the art to provide semiconductor packages in which a transistor or other die is mounted directly on a metallic substrate together with an insulated lead frame. In addition to mounting the die and the insulated lead frame, the substrate, which provides a further electrical connection for the die, acts as a heat spreader in order to provide desired heat removal. The die may be of the LDMOS (lateral diffusion metal oxide semiconductor) type and the package of the type for packaging LDMOS power transistors. A variety of different materials, metals and composites have been used in making the substrate, to the end that improved heat removal is constantly being sought. The substrate material must be chosen to have a low thermal expansion coefficient that matches those of the die and the insulator for the lead frame. Copper/tungsten composites are frequently used in the fabrication of such substrates.
While materials such as copper/tungsten composites have proven to be well suited for heat removal from the semiconductor package, these and other conventional designs still leave room for improvement, particularly in terms of their ability to remove heat during operation of the structure.
The present invention provides improved semiconductor packaging structures and methods of making such structures. More particularly, the present invention provides improved substrate structures for mounting a die and an insulated lead frame thereon. The substrate structures provide for improved heat removal over the prior art structures, and have a desirably low thermal expansion coefficient. Further in accordance with the invention, advantageous methods are provided for making the substrates.
Semiconductor packages in accordance with the invention comprise a metallic substrate and a die mounted directly thereon, with the substrate including a body having opposite sides on which a pair of copper layers are formed, the die being mounted on one of the pair of copper layers, and the body being at least partially comprised of a copper/diamond composite. Diamonds have extremely high thermal conductivity and low thermal expansion so that when formed into a matrix with copper, an improved substrate is provided.
In a first embodiment according to the invention, the body is entirely comprised of a copper/diamond composite. In a second embodiment, the body is comprised of a copper/tungsten composite having a copper/diamond composite insert therein. In both cases, the semiconductor package includes an insulated lead frame coupled to one of the pair of copper layers adjacent the die. The lead frame comprises a plurality of leads coupled by bond wires to the die. Preferably, the body is of generally planar configuration and has a generally uniform thickness between opposite sides. The pair of copper layers are of relatively thin, generally planar configuration.
The copper/diamond composite is comprised of diamond particles within a copper matrix. The diamond particles may be coated with one or more of Cr, W, Mo, Co, Cu, Ti, Si, SiC, TiN, TiC, Ta and Zr. In a preferred embodiment, the diamond particles are coated with a layer of Cr, a layer of W, a layer of Co, and a layer of Cu.
In a preferred method of making a copper/diamond composite for use in semiconductor substrates, according to the invention, diamond particles are coated with multiple layers of elements or inorganic compounds. The coated particles are mixed with a dry-processing binder, and are compacted in a die under pressure to form a compacted body. The compacted body is placed on or under a piece of copper and heated in a vacuum or hydrogen atmosphere to evaporate or decompose the binder. The compacted body is then heated in a vacuum or hydrogen atmosphere to cause bonding or partial sintering of the coated diamond particles. The compacted body is then heated in a hydrogen atmosphere to a temperature slightly above the melting point of copper to melt and draw the copper into the bonded or partially sintered diamond particles. Following this, the compacted body is cooled, and is cut to a desired shape. Following cutting, copper layers may be bonded to opposite sides of the body such as by brazing.
To make a substrate in accordance with the second embodiment described above, a copper/tungsten composite is provided. The compacted body and the copper/tungsten composite are then heated to above the melting point of copper to integrate the compacted body with the copper/tungsten composite. The result is a substrate of copper/tungsten composite having a copper/diamond insert therein.
The step of cooling the compacted body may simply comprise cooling the body to room temperature. Alternatively, the step may comprise partially cooling the compacted body, establishing a low pressure atmosphere around the compacted body, and re-heating the compacted body to above the melting point of copper long enough to remove dissolved hydrogen from the copper.